A normal stop cock valve tap consists of a body with an inlet, an outlet, a valve seat between the inlet and outlet, a spindle with a handle and a valve member movable by the handle to either cause it to seal against the seat or release it for movement away from the seat.
With such valves there is no protection against backflow and harmful substances can travel into the supply line and contaminate liquid in the supply line which may exit from that of other valves coupled to the line.
Backflow prevention devices are known and these are either fitted upstream or downstream of the valve. Where the valve is a standard stop cock, downstream backflow prevention devices may either be fitted directly to the outlet of the stop cock or in a line attached to the outlet. Such devices can readily be removed and for this reason are undesirable.
Backflow prevention devices mounted upstream of the stop cock require fitting by a skilled tradesman and are difficult to retrofit.
Reference may also be made to Australian specification 68314/90 which describes a tap having the same construction as a conventional stop cock as described above which also includes a backflow preventer. In this arrangement an operator is connected to a valve closure member which opens and closes the flow passage between inlet and outlet. The backflow preventer comprises a tubular body having a flow path therethrough which is connected to the outlet and a non-return valve fitted within the tubular body. A vacuum break opening is formed in a wall of the tubular body and there is also provided a diaphragm to open the opening and permit communication between the atmosphere and the flow path when the pressure within the flow path is less than ambient pressure.
In Australian specification 69869/91 (EP441151) there is described a mixer tap with a shower head attachment which incorporates a venting device for preventing water from being drawn back via a hose connection between a housing of the mixer tap and shower head. There is provided a pair of valves inserted in a water feed for the shower head wherein each valve has a movable valve element displaced between two opposing valve seats respectively associated with a vent opening and a flow opening for the water feed. Each valve element is biased towards the valve seat associated with the water feed opening.
Australian specification 59050/90 describes a backflow preventer valve adopted to be fitted to a conventional tap or stopcock as described above. The valve has an inlet and outlet connected by one or more passages and an annular main valve member formed of flexible material which may move between a closed position when there is back flow through the valve and an open position when there is normal flow of liquid from upstream to downstream. One or more discharge passages connect to interior of the valve body to atmosphere on the downstream side of the main valve member. A spring loaded valve actuator closes the main valve member when there is a back flow from the downstream side and also closes the discharge passage(s) when there is a normal flow from upstream to downstream.
U.S. Pat. No. 4,969,483 describes an anti-siphon assembly especially for a shower mechanism which has an automatic flexible one way valve associated with a cover of a housing of the anti-siphon assembly. The cover has air intakes controlled by the one way valve. The valve has an elastic lip and remains closed while the pressure in a control valve of the shower mechanism is above atmospheric but opens as soon as the pressure drops below atmospheric in the control valve.
U.S. Pat. No. 4,909,270 describes an anti-siphon water backflow fitting which is fitted to a threaded neck portion of a faucet and which is adapted to receive a hose coupling. The fitting includes a pair of valves which are biased shut to prevent backflow of water into the faucet when a water source is turned off and which are open when the water source is turned on.
U.S. Pat. No. 4,804,010 describes a sink faucet assembly with an anti-siphon assembly forming a clamp for a sink deck.
U.S. Pat. No. 4,726,390 describes a faucet hose bib vacuum breaker or backflow preventer which includes a web with multiple vent holes and a peripherally supporting flexible resilient diaphragm and overlying plate.
U.S. Pat. No. 4,712,575 describes a self draining hose connection vacuum breaker which has a movable piston with two different positions dependent on whether or not the hose is fitted.
U.S. Pat. No. 4,712,574 describes a vacuum breaking valve for a pressurised fluid line which has a pipe tapped into a line upstream of an outlet terminating in a ball shaped housing. The housing receives a check valve including a movable diaphragm arranged for ensuring that fluid does not escape during normal operation of the line but admits vacuum breaking air in the event of line pressure loss.
U.S. Pat. No. 4,700,732 refers to a faucet drain preventing damage due to freezing having a lightly sprung ball valve allowing trapped water to drain when pressure is released.
U.S. Pat. No. 4,827,538 describes a mixing tap for a sink which has a mixing chamber with a relief valve located in a wall of the mixing chamber which is responsive to negative pressure. The relief valve comprises a closure member which is forced into a closed position by gravity and/or a spring.
U.S. Pat. Nos. 4,696,322 and 4,805,661 refers to a faucet back flow preventer valve assembly which includes a control valve housing which has a valve member comprising a ball with ports for selectively uncovering or covering a supply inlet and outlet. A check valve is mounted in an anti-siphon passageway within the ball and allows air into the ball from an auxiliary opening which communicates with atmosphere and hence into the supply inlet if negative pressure exists.
U.S. Pat. No. 4,669,497 refers to a water faucet backflow preventer assembly which includes a tubular body having a flow passageway having an inlet and outlet. The flow passageway includes an integral shoulder and a pair of exhaust ports upstream of the shoulder. A valve assembly located within the passageway is spring biased into a seated position closing communication between the outlet and inlet while permitting fluid communication between the outlet and the discharge ports for draining any reverse water flow. The valve assembly is yieldable to an operating position under the force of pressurised fluid flow into the inlet and to open communication with the outlet. A resilient valve washer contacts the integral shoulder to close communication between the passageway and the discharge ports to prevent water leakage.
United Kingdom specification 1602193 refers to an anti-siphoning device having a domed closure urged by atmospheric pressure to seal a mating aperture. The device is fitted into a garden water tap operated by a handle, a spindle and an outlet and is mounted in the outlet.
U.S. Pat. No. 4,134,419 refers to a combination faucet and anti-siphon valve having a body with two effluent inlets communicating with single outlet and central drinking water dispensing tube.
Having regard to the prior art described above which was located in an international search carried out in respect of the invention it will be appreciated that such prior art valve assemblies for preventing anti-syphoning or prevention of backflow were found to be deficient in operation because of their inability to operate effectively when installed in stop cocks or taps when the stop cock or tap was open or closed.